EUROPE is on the brink of war after Ukraine ordered a nationwide mobilisation of their troops ready for full-on combat with Russia.

The Ukrainian Prime Minister has accused Russian President Vladimir Putin of "declaring war" as Russian troops spurned Western pleas not to intervene and sent troops to the stricken country.

Today Britain pulled out of a G8 summit in Sochi saying that the territory of the Ukraine had been violated as hundreds of armed men surrounded a military base in the Crimea.

Foreign Secretary William Hague again appealed for calm in what is now Moscow’s biggest confrontation with the West since the Cold War.

"This is not a threat: this is actually the declaration of war to my country," Ukraine's Prime Minister Arseny Yatseniuk said in response to the Russian government giving its military the green light to invade.

Russian forces have already seized Crimea - a Black Sea sector of land which Moscow already uses as a naval base.

Soldiers are said to have marched through small military outposts and forced Ukrainian troops to stand down.

Russian soldiers stand guard at the local government headquarters in Crimea [REUTERS]

We call on both sides to ensure that there can be a reduction in tensions and the avoidance of conflict

William Hague

No shots have been fired yet but 150,000 armed troops wait for further instructions on the Ukraine border.

In response, the Ukraine has ordered its troops to be on high alert with Yatseniuk warning “we are on the brink of disaster.”

Ukraine's population of 46 million has divided loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the EU, while eastern and southern regions like Crimea look to Russia for support.

Russia has long wanted to reclaim the lush Crimean Peninsula, which is the size of Belgium and was part of Russia until 1954. It's Black Sea Fleet is stationed there and nearly 60 percent of Crimea's residents identify themselves as Russian.

Today western nations condemned the actions of Putin, warning they would go “to the hilt” to isolate it economically.

Protests around the world have taken place after Russia mobilised their troops [REUTERS]

British Foreign Minister William Hague, en route to the capital Kiev, said Britain would not attend the G8 summit planned later this week.

“We have to recognise the sovereignty and the territorial integrity of Ukraine has been violated.” he said.

"Our response is diplomatic and peaceful and it should be. That is our response but it will be a very united diplomatic response and not just from the Western world but from many other countries in the world and that is something that Russia will have to think hard about.

"We call on both sides to ensure that there can be a reduction in tensions and the avoidance of conflict."

US Secretary of State John Kerry on Sunday condemned Russia's "incredible act of aggression" in Ukraine and threatened "very serious repercussions" from the United States and other countries, including sanctions to isolate Russia economically.

"You just don't in the 21st century behave in 19th century fashion by invading another country on completely trumped up pre-text," Kerry told the CBS program "Face the Nation."

"It's an incredible act of aggression. It is really a stunning, willful choice by President (Vladimir) Putin to invade another country. Russia is in violation of the sovereignty of Ukraine. Russia is in violation of its international obligations," Kerry added.

He warned: “American businesses may well want to start thinking twice about whether they want to do business with a country that behaves like this."

"There are very serious repercussions that can flow out of this. There are a broad array of options that are available, not just to the United States but to our allies."

Russian troops wait on the border [EPA]

Meanwhile worldwide protests have taken place against Putin's advances in the Crimea.

British-based Ukrainians have gathered outside the Russian embassy in London holding up signs saying "Hands off Ukraine".

Car horns sounded in support of the 400-person group with the protesters calling for British support.

Protestor Bogdan Maksak attended the protest to call on Britain to uphold its commitment under the 1994 Budapest Memorandum and take action against Russia.

The agreement, signed by the UK, USA, Russia and Ukraine, promised to uphold the territorial integrity of Ukraine.

The 20-year-old student said: "I would like to ask the British Government to assist Ukraine and also keep its promises of the. Budapest Memorandum.

"They have to make sure that we are protected and they also have to take action because at this stage staying neutral is almost like playing on the Russian side."

Protests were made outside the White House in Washington D.C yesterday calling for US support while Kiev's Independence Square remains packed with Ukrainian demonstrators.